The goal of this project is to beter understand the phenomenon of neurite retraction. Retraction of axons occurs in embryogenesis and early postnatal life as neurons reduce their fields of innervation to achieve adult patterns of connectivity. Retraction may also occur pathologically in adults, leading to losses of normal synapses and to impaired functional capacities. This study will use a novel in vitro system that allows retraction to be synchronously induced in a population of growing nerve fibers. Specific aims of the study are to 1) establish the details of the rearrangements of cytoskeletal and membranous structures that accompany retraction, 2) determine whether structural components of the retracted neurite are neutilized for neurite growth and 3) probe the metabolic events that are required for retraction. Light microscopic observations of the retraction process will be augmented by flourescence microscopy of the formation and movements of endocytic vesicles labled by flurescent probes. In addition, alterations of cell-substratum adhesion during retraction will be examined with interference reflection microscopy. Electron microscopic observations will include determinations of the surface rearrangements of retracting neurites, and elucidation of the membrane compartments involved in the retrieval and reutilization of the plasma membrane of retracted neurites. The rearrangements during neurite retraction of the cytoskeleton and its reorganization for regrowth of neurites will also be examined electron microscopically.